POTASSIUM CYANIDE 83 flame) and by means of litmus paper (red coloration). The mica is recog- nised by its insolubility in water. 7. Quantitative Determination.—0-1-0-2 gram is distilled with hydrochloric acid and the chlorine absorbed in potassium iodide, the liberated iodine being titrated with thiosulphate solution : i c.c. N/io-thiosulphate = 0-0020416 gram KC1O3. POTASSIUM CHLORIDE See Stassfurt Salts, under Fertilisers POTASSIUM GHROMATE KaCr04 = 194-2 Yellow crystals, soluble in water. It is moderately pure as sold, the most frequent impurities being free alkalies and sulphates, detectable as follows : 1. Free Alkali.—The aqueous solution, suitably diluted, is tested with a few drops of phenolphthalein. 2. Sulphates.—3-5 grams in 100 c.c. of water, acidified with hydro- chloric acid, are tested with barium chloride : with the pure salt, no tur- bidity should be formed even after 12 hours. 3. Chlorides.—The dilute solution, heated with nitric acid and silver nitrate, should give no precipitate. 4. Alumina, Alkaline Earths.—The 1: 20 solution is tested with ammonia and ammonium oxalate. 5. Quantitative Determination.—See Potassium Bichromate. POTASSIUM CYANIDE KCN = 65-11 (65) White or dirty white powder or fused masses, very readily soluble in water to an alkaline solution and soluble also in alcohol if this is not too concentrated. The usual impurities are carbonates, sulphates, chlorides, sulphides, cyanates, thiocyanates, ferrocyanides and soda. Sometimes it may also contain small quantities of heavy metals, especially iron and lead, and traces of silver. The tests made are as follows : 1. Solubility.—i gram in 10 c.c. of water should give a clear solution, and 2 grams should dissolve at the ordinary temperature in 50 c.c. of 70% alcohol (by weight). 2. Carbonates.—'The aqueous solution (i: 10) is treated with lime water, which gives a white turbidity or precipitate in presence of carbonates. 3. Sulphates.—The i : 10 solution, acidified with hydrochloric acid (under a hood), is tested with barium chloride. 4. Sulphides.—The i : 10 solution is tested with lead acetate ; black coloration or precipitate in presence of sulphide.